High energy density batteries may develop an internal short circuit spontaneously or in response to overheating or any of a variety of different abusive conditions (e.g., manufacturing flaws, improper handling or storage, misuse, improper charging, etc.). This internal short leads to the release of energy, in the form of heat, within the cell. If the magnitude of the short is sufficiently large, the temperature of the cell may continue to increase until material decomposition and thermal runaway occurs. If the shorted cell undergoing thermal runaway is mounted within a battery pack, the large amount of thermal energy rapidly released during a thermal runaway event may cause other cells in proximity to the affected cell to enter into thermal runaway, leading to a cascading effect. As a result, power from the battery pack is interrupted and the system employing the battery pack is likely to incur extensive collateral damage due to the scale of the thermal runaway event and the associated release of thermal energy.
Although unlikely, the possibility that a cell within a battery pack may go into thermal runaway due to a spontaneous internal short has led to the need to test battery packs used in safety sensitive applications in order to determine their response characteristics to such an event. Battery pack testing is especially important given the recent surge in demand for electric vehicles, and thus large battery assemblies. Unfortunately, to date such testing has been hampered by the inability to intentionally induce a localized internal short without the test methodology itself compromising the accuracy of the results.
Utilizing conventional test equipment, inducement of a localized internal short is typically accomplished by either conductive overheating or cell overcharging. Conductive overheating requires that a heating coil or other resistive, high temperature rated heating element be located in close proximity to the cell, for example by wrapping the cell with the heating element. The heat is then transferred to the cell through thermal conduction. The advantage of this approach is that the tester is able to select the desired heating profile to be applied to the cell. Additionally, the state-of-charge and runaway character of the cell is preserved using this testing approach. Unfortunately this testing methodology dramatically alters the thermal environment of the cells within the battery pack by introducing extra thermal mass into the pack and, in some instances, requiring pack modification in order to accommodate the necessary heating elements. Also, as it is difficult to apply the heat only to the desired cell, adjacent cells may be excessively heated before the initiated thermal runaway event occurs, thereby further altering the thermal environment of the test pack.
The other approach often used to simulate a spontaneous internal cell short is by overcharging the cell in question. While this approach preserves the thermal environment surrounding the cells, unfortunately it changes the character of the thermal runaway reaction. In general, the reaction caused by overcharging is more energetic than would typically occur during an internal cell short. Furthermore, while a spontaneous internal short may develop with a variety of constant or profiled magnitudes, it is difficult to simulate internal shorts of various magnitudes by overcharging a single cell.
As prior art methods of testing a cell for thermal runaway unrealistically alter the character of the event or introduce extra mass or localized hot spots outside the cell that may not be representative of the operational thermal environment of the battery cell or module, what is needed is a method for inducing thermal runaway in a battery cell with minimal impact on the character of the event or the thermal environment of the surrounding cells. The present invention provides such a method.